1. Field of the Invention
This invention relates to methods of analyzing atomic species by means of their emission spectra and more particularly to analysis of atomic species by emission spectra wherein the atomic species are excited to emit light by means of excited metastable gas molecules. Still more particularly, it relates to analysis of atomic species by means of their emission spectra wherein atoms of the atomic species are excited by contact with excited metastable nitrogen gas molecules.
2. Description of the Prior Art
Qualitative and quantitative analysis of atomic species in the vapor phase by means of their absorption or emission spectra are well known analytical chemical techniques. In atomic absorption spectroscopy, a beam of light is passed through a vapor containing the atomic species to be analyzed and the amount of the species present is determined by the amount of light absorbed by the vapor. In emission spectroscopy, the atomic species in the vapor phase are excited to emit fluorescent radiation and the spectrum and intensity of the emitted light are analyzed to determine which species are present and the concentration of each. Various methods of exciting atomic species to emit fluorescent radiation have been used, such as arcs, sparks, and flames. It is also known to excite the atomic species by contact with metastable atoms of an excited, relatively inert gas in a flowing gaseous medium.
Capelle, U.S. Pat. No. 4,150,951, discloses a method of analyzing for trace amounts of metals and other fluorescing species in the gas phase by introducing the species to be analyzed into a gas stream containing an energetic metastable species of nitrogen or a noble gas. The species to be analyzed is excited by contact with the metastable exciting species and subsequently emits fluorescence at characteristic wavelengths. The metastable species are produced by subjecting a gas stream containing a noble gas or nitrogen at pressures of less than 10 Torr to a microwave discharge. The method of Capelle is disclosed to be not useful at relatively high concentrations of metal atoms (about 10.sup.13 atoms/cm.sup.3) because the amount of activating metastable species which can be produced by the microwave discharge will not adequately excite concentrations of metal atoms above this limit.
Taylor, U.S. Pat. No. 4,148,612, discloses a method of detecting and measuring trace impurities in a flowing gas system by mixing the gas with a second gas stream containing excited metastable species which transfer their energy to the trace impurities whereby the impurities themselves become excited and emit radiation. This emitted radiation is detected and analyzed by a conventional emission spectrometer, and the concentration of impurities may be determined in the usual way from the location and intensity of the lines in the emission spectrum. The method is disclosed as useful in analyzing any species which is capable of being excited by energy transfer from a metastable species and subsequently emitting the energy so acquired in the form of light in a spectral region where it may be detected.
Ault, U.S. Pat. No. 3,545,863, discloses a method for detecting trace amounts of mercury by excitation of the mercury vapor in a helium glow arc sustained by a glow discharge.
The known methods for analyzing atomic species in the gas phase, however, have suffered from a number of drawbacks.
Atomic absorption spectroscopy is capable of analyzing only a single element at a time, the apparatus required to vaporize the sample species is a rather complex apparatus, and a separate light source is required for the absorption measurement which requires a special lamp for each element to be analyzed.
The technique of choice used in the above Taylor and Capelle patents for generating the active metastable species is microwave discharge. Microwave discharge, in addition to requiring a relatively large, complex, and expensive apparatus, is a rather inefficient method of generating excited metastable species, particularly of nitrogen. This means that large amounts of power are dissipated, with attendant cooling requirements, and, most important, the actual concentrations of active species attained are rather small. This has the consequence that the available amount of metastable species is sufficient to excite only relatively small amounts of the sample species and, accordingly, only relatively small concentrations of the sample species can be accurately analyzed. The problems associated with this limited linear dynamic range are discussed by Capelle. Another difficulty with microwave discharges and other methods of generating excited metastable species which use a relatively large input of energy is that they produce, besides the desired metastable species, other luminescent species which contribute to the general background radiation observed by the detector ("noise") and may interfere with the emission from the desired sample species. This excess background radiation may limit the lowest concentration of sample species which can be accurately analyzed.
Microwave discharge, for instance, necessarily involves the generation of atomic nitrogen. Atomic nitrogen is a quencher for the metastable species such that when the metastable species comes into contact with atomic nitrogen, it spontaneously decays to the ground state. Thus, microwave generation is a poor source of metastable nitrogen. Moreover, while some of the nitrogen atoms will come together to form metastable nitrogen, much more of the nitrogen atoms will yield interfering higher excited species and will also react with oxygen atoms thereby considerably enhancing the spectral interference.
Hence, a need has continued to exist for a method of trace element analysis which has a high sensitivity and a large linear dynamic range, which is capable of analyzing more than a single element at a time, and which uses relatively simple and convenient apparatus.